The use of polyalkylene glycol esters in lubricating oil compositions

ABSTRACT

The presently claimed invention is directed to the use of polyalkylene glycol ester that are prepared by reacting polytetrahydrofuran with at least one C 9 -C 27  acid in lubricating oil compositions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. §371) of PCT/EP2014/074595, filed Nov. 14, 2014, which claims benefit of European Application No. 13194478.7, filed Nov. 26, 2013, both of which are incorporated herein by reference in their entirety.

The presently claimed invention is directed to the use of polyalkylene glycol ester that are prepared by reacting polytetrahydrofurane with at least one C₉-C₂₇ acid in lubricating oil compositions.

Lubricating oil compositions are used in a variety of applications, such as industrial applications, transportation and engines. Industrial applications comprise of applications such as hydraulic oil, air compressor oil, gas compressor oil, gear oil, bearing and circulating system oil, refrigerator compressor oil and steam and gas turbine oils.

Conventional lubricating oil compositions comprise base stocks, co-solvents and additives. The base stock is in each case selected according to the viscosity that is desired in the envisioned application. Combinations of base stocks of different viscosities, i.e. low and high viscosity respectively, are often used to adjust the needed final viscosity. The co-solvents are used to dissolve polar additives in usually less polar or unpolar base stocks.

The most common additives are antioxidants, detergents, anti-wear additives, metal deactivator, corrosion inhibitors, friction modifiers, extreme-pressure additives, defoamers, anti-foaming agents, viscosity index improvers and demulsifying agents. These additives are used to impart further advantageous properties to the lubricating oil composition including longer stability and additional protection.

However, after a certain operation time, lubricating oil compositions have to be replaced for various reasons such as lubricity loss and/or product degradation. Depending on the machine (engine, gearbox, compressor . . . ) engineering design and the affinity of the lubricant components to adhere to the surface, a certain residue of the lubricating oil composition (hold-up) remains in the machine, engine, gear etc. it is used in. When being replaced by an unused and possibly different lubricating oil composition, the used and new lubricants are mixed with each other. Thus, in order to avoid any complications during operation, compatibility between the old and new lubricant is very important.

Depending on their chemical properties a variety of components of lubricating oil compositions are incompatible with each other, i.e. the mixture of these components leads to oil gelling, phase separation, solidifying or foaming. The oil gelling leads to a dramatic increase of the viscosity which in turn can cause engine problems and can even require the engine to be replaced, if the damage is severe. Hence, when providing novel compounds that are used in lubricating oil compositions it should always be ensured that these compounds are compatible with compounds that are conventionally used in lubricating oil compositions.

Besides compatibility with other lubricants, another area of concern is the energy efficiency. The efficiency can be increased if losses are minimized. The losses can be categorized in losses without and with load, their sum being the total losses. Within many parameters which can be influenced by geometry, material etc. lubricant viscosity has a major effect on losses without load, i.e. spilling: Losses with load can be influenced by a low friction coefficient. Thus, at a given viscosity, energy efficiency strongly depends on the friction coefficient measured for a lubricant.

The friction coefficient can be measured with several methods like Mini-Traction-Machine (MTM), SRV, 2 disc test rig etc. The benefit of a MTM is that one can see the coefficient of friction as an influence of the slide roll ratio. Slide roll ratio describes the difference of the speeds of ball and disc used in the MTM.

EP 0 193 870 A2 discloses cold rolling mill lubricants for steel sheets containing polyalkylene glycol esters that are prepared from polyethylene glycol and linear fatty acids.

DE 195 1 493 C1 describes compositions comprising at least one base oil which is a polyether and a thickener.

JP Patent Appl. Publ. No. 2005-232434 and JP Patent Appl. Publ. No. 2008-7741 describe a bearing lubricant comprising polyalkylene glycol ester derived from linear or branched saturated aliphatic monocarboxylic acid and polyalkylene glycol.

U.S. Pat. No. 5,744,432 discloses polyester which are the reaction product of one or more carboxylic acids and a polyalkylene glycol. These polyester are used in aqueous-based stamping lubricants.

Thus, it was an objective of the presently claimed invention to provide compounds that show a low friction coefficient and that are compatible with base stocks, in particular base stocks such as mineral oils and polyalphaolefins, which are conventionally used in lubricating oil compositions for the preparation of lubricating oil compositions.

Surprisingly, it has been found that polyalkylene glycol ester that are prepared by reacting polytetrahydrofurane with at least one C₉-C₂₇ acid show a low friction coefficient and are compatible with base stocks that are conventionally used in lubricating oil compositions such as mineral oils and polyalphaolefins, preferably low viscosity polyalphaolefins, and consequently can be used for the formulation of lubricating oil compositions.

Hence, in one embodiment, the presently claimed invention is directed to the use of polyalkylene glycol ester of general formula (I)

-   wherein -   k is an integer in the range of ≧2 to ≦30, and -   R¹ and R², identical or different, each denote an unsubstituted,     branched alkyl radical having 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,     18, 19, 20, 21, 22, 23, 24, 25 or 26 carbon atoms, -   as lubricant.

By the term “lubricant”, in the sense of the presently claimed invention, is meant a substance capable of reducing friction between surfaces.

As used herein, “branched” denotes a chain of atoms with one or more side chains attached to it. Branching occurs by the replacement of a substituent, e.g., a hydrogen atom, with a covalently bonded alkyl radical.

“Alkyl radical” denotes a moiety constituted solely of atoms of carbon and of hydrogen without any kind of unsaturation, e.g. double bonds and/or triple bonds.

In other words, a branched alkyl radical contains at least one branched carbon atom, more preferably 1 or 2 or 3 branched carbon atoms, even more preferably 1 or 2 branched carbon atoms, most preferably 1 branched carbon atom. Within the scope of the present invention “branched carbon atom” means a carbon atom in an alkyl radical (3° carbon atoms) which is bonded to three additional carbon atoms, and a carbon atom in an alkyl radical (4° carbon atoms) which is bonded to four additional carbon atoms.

The inventively claimed polyalkylene glycol esters of general formula (I) are oil soluble, which means that, when mixed with mineral oils and/or polyalphaolefins, preferably low viscosity polyalphaolefins, in a weight ratio of 10:90, 50:50 and 90:10, the inventively claimed polyalkylene glycol esters of general formula (I) do not show phase separation after standing for 24 hours at room temperature for at least two weight rations out of the three weight ratios 10:90, 50:50 and 90:10.

Preferably the polyalkylene glycol esters of general formula (I) have a kinematic viscosity in the range of ≧30 mm²/s to ≦300 mm²/s, more preferably in the range of ≧30 mm²/s to ≦280 mm²/s, at 40° C., determined according to ASTM D 445.

Preferably the polyalkylene glycol esters of general formula (I) have a kinematic viscosity in the range of ≧5 mm²/s to ≦60 mm²/s, more preferably in the range of ≧7 mm²/s to ≦55 mm²/s, at 100° C., determined according to ASTM D 445.

Preferably the polyalkylene glycol esters of general formula (I) have a pour point in the range of ≧−40° C. to ≦0° C., more preferably in the range of ≧−30° C. to ≦0° C., determined according to DIN ISO 3016.

Preferably the polyalkylene glycol esters of general formula (I) have a weight average molecular weight Mw in the range of 500 to 4500 g/mol, more preferably in the range of 500 to 4000 g/mol, most preferably in the range of 600 to 3500 g/mol, even more preferably in the range of 700 to 3500 g/mol determined, determined according to DIN 55672-1.

Preferably the polyalkylene glycol esters of general formula (I) have a number average molecular weight Mn in the range of 400 to 4000 g/mol, more preferably in the range of 500 to 3500 g/mol, most preferably in the range of 500 to 3000 g/mol, even more preferably in the range of 500 to 2500 g/mol determined, determined according to DIN 55672-1.

Preferably the polyalkylene glycol esters of general formula (I) have a polydispersity in the range of 1.1 to 2.0, more preferably in the range of 1.1 to 1.9, most preferably in the range of 1.1 to 1.8, determined according to DIN 55672-1.

Preferably the polyalkylene glycol esters of general formula (I) have a total acid number in the range of ≧0.01 to ≦2.5 mg KOH/g, more preferably in the range of ≧0.01 to ≦2.4 mg KOH/g, most preferably in the range of ≧0.05 to ≦2.2 mg KOH/g determined according to ASTM D664.

Preferably the polyalkylene glycol esters of general formula (I) have a hydroxyl number in the range of ≧0.1 to ≦20 mg KOH/g, determined according to DIN 53240.

Preferably k is an integer in the range of ≧3 to ≦25, more preferably k is an integer in the range of ≧3 to ≦20, most preferably in the range of ≧5 to ≦20, even more preferably in the range of ≧6 to ≦16.

Preferably R¹ and R², identical or different, each denote an unsubstituted, branched alkyl radical having 13, 14, 15, 16, 17, 18, 19, 20, 21 or 22 carbon atoms. More preferably R¹ and R², identical or different, each denote an unsubstituted, branched alkyl radical having 15, 16, 17, 18, 19 or 20 carbon atoms.

Preferably R¹ and R² are identical and each denote an unsubstituted, branched alkyl radical having 13, 14, 15, 16, 17, 18, 19, 20, 21 or 22 carbon atoms. More preferably R¹ and R² are identical and each denote an unsubstituted, branched alkyl radical having 15, 16, 17, 18, 19 or 20 carbon atoms.

Preferably R¹ and R², identical or different, more preferably identical, each denote a moiety selected from the group consisting of

whereby n is an integer in the range of ≧8 to ≦23,

whereby m is an integer in the range of ≧7 to ≦22,

whereby o is an integer in the range of ≧1 to ≦6, p is an integer in the range of ≧2 to ≦19, and (o+p) is an integer in the range of ≧8 to ≦21,

and

whereby Q is an integer in the range of ≧1 to ≦6, R is an integer in the range of ≧2 to ≦19, and (Q+R) is an integer in the range of ≧8 to ≦20.

In another preferred embodiment the presently claimed invention is directed to the use of an polyalkylene glycol esters of general formula (I)

wherein

k is an integer in the range of ≧3 to ≦25,

and

R¹ and R², identical, each denote a moiety selected from the group consisting of

whereby n is an integer in the range of ≧10 to ≦18,

as a lubricant.

In another preferred embodiment the presently claimed invention is directed to the use of an polyalkylene glycol esters of general formula (I)

wherein

k is an integer in the range of ≧3 to ≦20,

and

R¹ and R², identical, each denote a moiety selected from the group consisting of

whereby n is an integer in the range of ≧11 to ≦16,

as a lubricant.

The polyalkylene glycol ester of general formula (I) are obtained by reacting at least one polytetrahydrofurane block polymer with at least one C₉-C₂₇ acid, optionally in the presence of at least one catalyst. Preferably the polyalkylene glycol ester of general formula (I) are obtained by reacting at least one polytetrahydrofurane block polymer with at least one C₉-C₂₇ acid in the presence of at least one catalyst.

Preferably the at least one C₉-C₂₇ acid is selected from the group consisting of 4,7-dimethylnonanoic acid, 4,8-dimethyl-decanoic acid, 8-methyl-undecanoic acid, iso-dodecanoic acid, 4,8-dimethyl-undecanoic acid, 4,9-dimethyl-undecanoic acid, iso-tridecanoic acid, 4,10-dimethyl-dodecanoic acid, iso-tetradecanoic acid, 4,11-dimethyl-tridecanoic acid, isopentadecanoic acid, 8,10-dimethyl-tetradecanoic acid, 4,12-dimethyl-tetradecanoic acid, iso-hexadecanoic acid, 4,11-dimethyl-pentadecanoic acid, 4,13-dimethyl-pentadecanoic acid, iso-heptadecanoic acid, anteiso heptadecanoic acid, iso-octadecanoic acid, iso-eicosanoic acid, iso-dodecanoic acid, iso-tetracosanoic acid, iso-pentacosanoic acid and isohexacosanoic acid.

Preferably the at least one catalyst is a catalyst selected from the group consisting of magnesium carbonate, zinc carbonate, zinc borate, tin(II) acetate, tin(II) octanoate, tin(II) lactate, zinc acetate, aluminum acetate, tin(II) trifluoromethane sulfonate, zinc trifluoromethane sulfonate, magnesium trifluoromethane sulfonate, tin(II) methane sulfonate, tin(II) p-toluene sulfonate, dibutyltin dilaurate (DBTL), antimony oxide (Sb₂O₃), butyl titanate, isopropyl titanate, acetic acid, methane sulfonic acid, ethane sulfonic acid, 1-propane sulfonic acid, 1-butane sulfonic acid, trifluoromethane sulfonic acid, benzene sulfonic acid, p-toluene sulfonic acid, pxylene-2-sulfonic acid, naphthalene-1-sulfonic acid, naphthalene 2-sulfonic acid, hydrochloric acid, sulfuric acid, phosphoric acid, triethylamine, pyridine, dimethyl aminopyridine, histidine, imidazoles, 1,8-Diazabicyclo[5.4.0]undec-7-en (DBU) and 1,5-Diazabicyclo(4.3,0)non-5-ene (DBN). More preferably the at least one catalyst is tin(II) octanoate or p-toluene sulfonic acid.

In another embodiment, the presently claimed invention is directed to the use of at least one polyalkylene glycol esters of general formula (I) as defined above or a mixture of polyalkylene glycol esters of general formula (I) as defined above for the preparation of a lubricating oil composition.

In another embodiment, the presently claimed invention is directed to a lubricating oil composition comprising at least one polyalkylene glycol esters of general formula (I) as defined above or a mixture of polyalkylene glycol esters of general formula (I) as defined above. Preferably the lubricating oil composition comprises 1% to ≦10% by weight or 1% to ≦40% by weight or ≧20% to ≦100% by weight,

more preferably ≧1% to ≦5% by weight or ≧1% to ≦35% by weight or ≧25% to ≦100% by weight,

most preferably ≧1% to ≦2% by weight or ≧2% to ≦30% by weight or ≧30% to ≦100% by weight,

of at least one polyalkylene glycol esters of general formula (I) as defined above, related to the total amount of the lubricating oil composition.

Preferably, the lubricating oil composition according to the presently claimed invention has a friction coefficient in the range of 0.003 to 0.030, more preferably in the range of 0.003 to 0.020, at 25% slide roll ratio (SRR) determined using mini-traction machine (MTM) measurements at 70° C. and 1 GPa.

In another embodiment, the presently claimed invention relates to an industrial oil comprising at least one polyalkylene glycol esters of general formula (I) as defined above.

Lubricating oil compositions comprising at least one polyalkylene glycol esters of general formula (I) as defined above or a mixture of polyalkylene glycol esters of general formula (I) as defined above can be used for various applications such as light, medium and heavy duty engine oils, industrial engine oils, marine engine oils, automotive engine oils, crankshaft oils, compressor oils, refrigerator oils, hydrocarbon compressor oils, very low-temperature lubricating oils and fats, high temperature lubricating oils and fats, wire rope lubricants, textile machine oils, refrigerator oils, aviation and aerospace lubricants, aviation turbine oils, transmission oils, gas turbine oils, spindle oils, spin oils, traction fluids, transmission oils, plastic transmission oils, passenger car transmission oils, truck transmission oils, industrial transmission oils, industrial gear oils, insulating oils, instrument oils, brake fluids, transmission liquids, shock absorber oils, heat distribution medium oils, transformer oils, fats, chain oils, minimum quantity lubricants for metalworking operations, oil to the warm and cold working, oil for water-based metalworking liquids, oil for neat oil metalworking fluids, oil for semi-synthetic metalworking fluids, oil for synthetic metalworking fluids, drilling detergents for the soil exploration, hydraulic oils, in biodegradable lubricants or lubricating greases or waxes, chain saw oils, release agents, moulding fluids, gun, pistol and rifle lubricants or watch lubricants and food grade approved lubricants.

A lubricating oil composition can comprise base stocks, co-solvents and a variety of different additives in varying ratios.

Preferably the lubricating oil composition further comprises base stocks selected from the group consisting of mineral oils (Group I, II or III oils), polyalphaolefins (Group IV oils), polymerized and interpolymerized olefins, alkyl naphthalenes, alkylene oxide polymers, silicone oils, phosphate esters and carboxylic acid esters (Group V oils). Preferably the lubricating oil comprises ≧50% to ≦99% by weight or ≧80% to ≦99% by weight or ≧90% to ≦99% by weight base stocks, related to the total amount of the lubricating oil composition.

Definitions for the base stocks in this invention are the same as those found in the American Petroleum Institute (API) publication “Engine Oil Licensing and Certification System”, Industry Services Department, Fourteenth Edition, December 1996, Addendum 1, December 1998. Said publication categorizes base stocks as follows:

a) Group I base stocks contain less than 90 percent saturates and/or greater than 0.03 percent sulphur and have a viscosity index greater than or equal to 80 and less than 120 using the test methods specified in the following table

b) Group II base stocks contain greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulphur and have a viscosity index greater than or equal to 80 and less than 120 using the test methods specified in the following table

c) Group III base stocks contain greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulphur and have a viscosity index greater than or equal to 120 using the test methods specified in the following table

Analytical Methods for Base Stock

Property Test Method Saturates ASTM D 2007 Viscosity ASTM D 2270 Index Sulphur ASTM D 2622 ASTM D 4294 ASTM D 4927 ASTM D 3120

Group IV base stocks contain polyalphaolefins. Synthetic lower viscosity fluids suitable for the present invention include the polyalphaolefins (PAOs) and the synthetic oils from the hydrocracking or hydroisomerization of Fischer Tropsch high boiling fractions including waxes. These are both stocks comprised of saturates with low impurity levels consistent with their synthetic origin. The hydroisomerized Fischer Tropsch waxes are highly suitable base stocks, comprising saturated components of iso-paraffinic character (resulting from the isomerization of the predominantly n-paraffins of the Fischer Tropsch waxes) which give a good blend of high viscosity index and low pour point. Processes for the hydroisomerization of Fischer Tropsch waxes are described in U.S. Pat. Nos. 5,362,378; 5,565,086; 5,246,566 and 5,135,638, as well in EP 710710, EP 321302 and EP 321304.

Polyalphaolefins suitable for the present invention, as either lower viscosity or high viscosity fluids depending on their specific properties, include known PAO materials which typically comprise relatively low molecular weight hydrogenated polymers or oligomers of alphaolefins which include but are not limited to C₂ to about C₃₂ alphaolefins with the C₈ to about C₁₆ alphaolefins, such as 1-octene, 1-decene, 1-dodecene and the like being preferred. The preferred polyalphaolefins are poly-1-octene, poly-1-decene, and poly-1-dodecene, although the dimers of higher olefins in the range of C₁₄ to C₁₈ provide low viscosity base stocks.

Low viscosity PAO fluids suitable for the present invention, may be conveniently made by the polymerization of an alphaolefin in the presence of a polymerization catalyst such as the Friedel-Crafts catalysts including, for example, aluminum trichloride, boron trifluoride or complexes of boron trifluoride with water, alcohols such as ethanol, propanol or butanol, carboxylic acids or esters such as ethyl acetate or ethyl propionate. For example, the methods disclosed by U.S. Pat. No. 3,149,178 or 3,382,291 may be conveniently used herein. Other descriptions of PAO synthesis are found in the following U.S. Pat. No. 3,742,082 (Brennan); U.S. Pat. No. 3,769,363 (Brennan); U.S. Pat. No. 3,876,720 (Heilman); U.S. Pat. No. 4,239,930 (Allphin); U.S. Pat. No. 4,367,352 (Watts); U.S. Pat. No. 4,413,156 (Watts); U.S. Pat. No. 4,434,308 (Larkin); U.S. Pat. No. 4,910,355 (Shubkin); U.S. Pat. No. 4,956,122 (Watts); and U.S. Pat. No. 5,068,487 (Theriot).

Group V base stocks contain any base stocks not described by Groups I to IV. Examples of Group V base stocks include alkyl naphthalenes, alkylene oxide polymers, silicone oils, phosphate esters and carboxylic acid esters.

Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-hexenes), poly(1-octenes), poly(1-decenes)); alkylbenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di(2-ethylhexyl)benzenes); polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenols); and alkylated diphenyl ethers and alkylated diphenyl sulphides and derivative, analogs and homologs thereof.

Further carboxylic acid esters suitable for the present invention include the esters of mono and polybasic acids with monoalkanols (simple esters) or with mixtures of mono and polyalkanols (complex esters), and the polyol esters of monocarboxylic acids (simple esters), or mixtures of mono and polycarboxylic acids (complex esters). Esters of the mono/polybasic type include, for example, the esters of monocarboxylic acids such as heptanoic acid, and dicarboxylic acids such as phthalic acid, succinic acid, alkyl succinic acid, alkenyl succinic acid, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkyl malonic acid, alkenyl malonic acid, etc., with a variety of alcohols such as butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, or mixtures thereof with polyalkanols, etc. Specific examples of these types of esters include nonyl heptanoate, dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, dibutyl-TMP-adipate, etc.

Also suitable for the present invention are esters, such as those obtained by reacting one or more polyhydric alcohols, preferably the hindered polyols such as the neopentyl polyols, e.g. neopentyl glycol, trimethylol ethane, 2-methyl-2-propyl-1,3-propanediol, trimethylol propane, trimethylol butane, pentaerythritol and dipentaerythritol with monocarboxylic acids containing at least 4 carbons, normally the C₅ to C₃₀ acids such as saturated straight chain fatty acids including caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid, and behenic acid, or the corresponding branched chain fatty acids or unsaturated fatty acids such as oleic acid, or mixtures thereof, with polycarboxylic acids.

Alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc., constitute another class of known synthetic lubricating oils. These are exemplified by polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide, and the alkyl and aryl ethers of polyoxyalkylene polymers (e.g., methyl-polyiso-propylene glycol ether having a molecular weight of 1000 or diphenyl ether of poly-ethylene glycol having a molecular weight of 1000 to 1500); and mono- and polycarboxylic esters thereof, for example, the acetic acid esters, mixed C₃-C₈ fatty acid esters and C₁₃ Oxo acid diester of tetraethylene glycol.

Silicon-based oils such as the polyalkyl-, polyaryl-, polyalkoxy- or polyaryloxysilicone oils and silicate oils comprise another useful class of synthetic lubricants; such oils include tetraethyl silicate, tetraisopropyl silicate, tetra-(2-ethylhexyl)silicate, tetra-(4-methyl-2-ethylhexyl)silicate, tetra-(p-tert-butyl-phenyl) silicate, hexa-(4-methyl-2-ethylhexyl)disiloxane, oly(methyl)siloxanes and poly(methylphenyl)siloxanes. Other synthetic lubricating oils include liquid esters of phosphorous-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid) and polymeric tetrahydrofurans.

The lubricating oil composition of the invention optionally further includes at least one other performance additive. The other performance additives include dispersants, metal deactivators, detergents, viscosity modifiers, extreme pressure agents (typically boron- and/or sulphur- and/or phosphorus-containing), antiwear agents, antioxidants (such as hindered phenols, aminic antioxidants or molybdenum compounds), corrosion inhibitors, foam inhibitors, demulsifiers, pour point depressants, seal swelling agents, friction modifiers and mixtures thereof.

The total combined amount of the other performance additives (excluding the viscosity modifiers) present on an oil free basis may include ranges of 0% by weight to 25% by weight, or 0.01% by weight to 20% by weight, or 0.1% by weight to 15% by weight or 0.5% by weight to 10% by weight, or 1 to 5% by weight of the composition.

Although one or more of the other performance additives may be present, it is common for the other performance additives to be present in different amounts relative to each other.

In one embodiment the lubricating composition further includes one or more viscosity modifiers.

When present the viscosity modifier may be present in an amount of 0.5% by weight to 70% by weight, 1% by weight to 60% by weight, or 5% by weight to 50% by weight, or 10% by weight to 50% by weight of the lubricating composition.

Viscosity modifiers include (a) polymethacrylates, (b) esterified copolymers of (II) a vinyl aromatic monomer and (ii) an unsaturated carboxylic acid, anhydride, or derivatives thereof, (c) esterified interpolymers of (II) an alpha-olefin; and (ii) an unsaturated carboxylic acid, anhydride, or derivatives thereof, or (d) hydrogenated copolymers of styrene-butadiene, (e) ethylene-propylene copolymers, (f) polyisobutenes, (g) hydrogenated styrene-isoprene polymers, (h) hydrogenated isoprene polymers, or (II) mixtures thereof.

In one embodiment the viscosity modifier includes (a) a polymethacrylate, (b) an esterified copolymer of (II) a vinyl aromatic monomer; and (ii) an unsaturated carboxylic acid, anhydride, or derivatives thereof, (c) an esterified interpolymer of (II) an alpha-olefin; and (ii) an unsaturated carboxylic acid, anhydride, or derivatives thereof, or (d) mixtures thereof.

Extreme pressure agents include compounds containing boron and/or sulphur and/or phosphorus.

The extreme pressure agent may be present in the lubricating composition at 0% by weight to 20% by weight, or 0.05% by weight to 10% by weight, or 0.1% by weight to 8% by weight of the lubricating composition.

In one embodiment the extreme pressure agent is a sulphur-containing compound. In one embodiment the sulphur-containing compound may be a sulphurised olefin, a polysulphide, or mixtures thereof. Examples of the sulphurised olefin include a sulphurised olefin derived from propylene, isobutylene, pentene; an organic sulphide and/or polysulphide including benzyldisulphide; bis-(chlorobenzyl) disulphide; dibutyl tetrasulphide; di-tertiary butyl polysulphide; and sulphurised methyl ester of oleic acid, a sulphurised alkylphenol, a sulphurised dipentene, a sulphurised terpene, a sulphurised Diels-Alder adduct, an alkyl sulphenyl N′N-dialkyl dithiocarbamates; or mixtures thereof.

In one embodiment the sulphurised olefin includes a sulphurised olefin derived from propylene, isobutylene, pentene or mixtures thereof.

In one embodiment the extreme pressure agent sulphur-containing compound includes a dimercaptothiadiazole or derivative, or mixtures thereof. Examples of the dimercaptothiadiazole include compounds such as 2,5-dimercapto-1,3,4-thiadiazole or a hydrocarbyl-substituted 2,5-dimercapto-1,3,4-thiadiazole, or oligomers thereof. The oligomers of hydrocarbyl-substituted 2,5-dimercapto-1,3,4-thiadiazole typically form by forming a sulphur-sulphur bond between 2,5-dimercapto-1,3,4-thiadiazole units to form derivatives or oligomers of two or more of said thiadiazole units. Suitable 2,5-dimercapto-1,3,4-thiadiazole derived compounds include for example 2,5-bis(tert-nonyldithio)-1,3,4-thiadiazole or 2-tert-nonyldithio-5-mercapto-1,3,4-thiadiazole. The number of carbon atoms on the hydrocarbyl substituents of the hydrocarbyl-substituted 2,5-dimercapto-1,3,4-thiadiazole typically include 1 to 30, or 2 to 20, or 3 to 16.

In one embodiment the dimercaptothiadiazole may be a thiadiazole-functionalised dispersant. A detailed description of the thiadiazole-functionalised dispersant is described is paragraphs [0028] to [0052] of International Publication WO 2008/014315.

The thiadiazole-functionalised dispersant may be prepared by a method including heating, reacting or complexing a thiadiazole compound with a dispersant substrate. The thiadiazole compound may be covalently bonded, salted, complexed or otherwise solubilised with a dispersant, or mixtures thereof.

The relative amounts of the dispersant substrate and the thiadiazole used to prepare the thiadiazole-functionalised dispersant may vary. In one embodiment the thiadiazole compound is present at 0.1 to 10 parts by weight relative to 100 parts by weight of the dispersant substrate. In different embodiments the thiadiazole compound is present at greater than 0.1 to 9, or greater than 0.1 to less than 5, or 0.2 to less than 5: to 100 parts by weight of the dispersant substrate. The relative amounts of the thiadiazole compound to the dispersant substrate may also be expressed as (0.1-10):100, or (>0.1-9):100, (such as (>0.5-9):100), or (0.1 to less than 5):100, or (0.2 to less than 5):100.

In one embodiment the dispersant substrate is present at 0.1 to 10 parts by weight relative to 1 part by weight of the thiadiazole compound. In different embodiments the dispersant substrate is present at greater than 0.1 to 9, or greater than 0.1 to less than 5, or about 0.2 to less than 5: to 1 part by weight of the thiadiazole compound. The relative amounts of the dispersant substrate to the thiadiazole compound may also be expressed as (0.1-10):1, or (>0.1-9):1, (such as (>0.5-9):1), or (0.1 to less than 5):1, or (0.2 to less than 5):1.

The thiadiazole-functionalised dispersant may be derived from a substrate that includes a succinimide dispersant (for example, N-substituted long chain alkenyl succinimides, typically a polyisobutylene succinimide), a Mannich dispersant, an ester-containing dispersant, a condensation product of a fatty hydrocarbyl monocarboxylic acylating agent with an amine or ammonia, an alkyl amino phenol dispersant, a hydrocarbyl-amine dispersant, a polyether dispersant, a polyetheramine dispersant, a viscosity modifier containing dispersant functionality (for example polymeric viscosity index modifiers (VMs) containing dispersant functionality), or mixtures thereof. In one embodiment the dispersant substrate includes a succinimide dispersant, an ester-containing dispersant or a Mannich dispersant.

In one embodiment the extreme pressure agent includes a boron-containing compound. The boron-containing compound includes a borate ester (which in some embodiments may also be referred to as a borated epoxide), a borated alcohol, a borated dispersant, a borated phospholipid or mixtures thereof. In one embodiment the boron-containing compound may be a borate ester or a borated alcohol.

The borate ester may be prepared by the reaction of a boron compound and at least one compound selected from epoxy compounds, halohydrin compounds, epihalohydrin compounds, alcohols and mixtures thereof. The alcohols include dihydric alcohols, trihydric alcohols or higher alcohols, with the proviso for one embodiment that hydroxyl groups are on adjacent carbon atoms, i.e., vicinal.

Boron compounds suitable for preparing the borate ester include the various forms selected from the group consisting of boric acid (including metaboric acid, orthoboric acid and tetraboric acid), boric oxide, boron trioxide and alkyl borates. The borate ester may also be prepared from boron halides.

In one embodiment suitable borate ester compounds include tripropyl borate, tributyl borate, tripentyl borate, trihexyl borate, triheptyl borate, trioctyl borate, trinonyl borate and tridecyl borate. In one embodiment the borate ester compounds include tributyl borate, tri-2-ethylhexyl borate or mixtures thereof.

In one embodiment, the boron-containing compound is a borated dispersant, typically derived from an N-substituted long chain alkenyl succinimide. In one embodiment the borated dispersant includes a polyisobutylene succinimide. Borated dispersants are described in more detail in U.S. Pat. No. 3,087,936; and U.S. Pat. No. 3,254,025.

In one embodiment the borated dispersant may be used in combination with a sulphur-containing compound or a borate ester.

In one embodiment the extreme pressure agent is other than a borated dispersant.

The number average molecular weight of the hydrocarbon from which the long chain alkenyl group was derived includes ranges of 350 to 5000, or 500 to 3000, or 550 to 1500. The long chain alkenyl group may have a number average molecular weight of 550, or 750, or 950 to 1000.

The N-substituted long chain alkenyl succinimides are borated using a variety of agents including boric acid (for example, metaboric acid, orthoboric acid and tetraboric acid), boric oxide, boron trioxide, and alkyl borates. In one embodiment the borating agent is boric acid which may be used alone or in combination with other borating agents.

The borated dispersant may be prepared by blending the boron compound and the N-substituted long chain alkenyl succinimides and heating them at a suitable temperature, such as, 80° C. to 250° C., or 90° C. to 230° C., or 100° C. to 210° C., until the desired reaction has occurred. The molar ratio of the boron compounds to the N-substituted long chain alkenyl succinimides may have ranges including 10:1 to 1:4, or 4:1 to 1:3; or the molar ratio of the boron compounds to the N-substituted long chain alkenyl succinimides may be 1:2. Alternatively, the ratio of moles B:moles N (that is, atoms of B:atoms of N) in the borated dispersant may be 0.25:1 to 10:1 or 0.33:1 to 4:1 or 0.2:1 to 1.5:1, or 0.25:1 to 1.3:1 or 0.8:1 to 1.2:1 or about 0.5:1 An inert liquid may be used in performing the reaction. The liquid may include toluene, xylene, chlorobenzene, dimethylformamide or mixtures thereof.

In one embodiment the lubricating composition further includes a borated phospholipid. The borated phospholipid may be derived from boronation of a phospholipid (for example boronation may be carried out with boric acid). Phospholipids and lecithins are described in detail in Encyclopedia of Chemical Technology, Kirk and Othmer, 3rd Edition, in “Fats and Fatty Oils”, Volume 9, pages 795-831 and in “Lecithins”, Volume 14, pages 250-269.

The phospholipid may be any lipid containing a phosphoric acid, such as lecithin or cephalin, or derivatives thereof. Examples of phospholipids include phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine, phosphotidic acid and mixtures thereof. The phospholipids may be glycerophospholipids, glycerol derivatives of the above list of phospholipids. Typically, the glycerophospholipids have one or two acyl, alkyl or alkenyl groups on a glycerol residue. The alkyl or alkenyl groups may contain 8 to 30, or 8 to 25, or 12 to 24 carbon atoms. Examples of suitable alkyl or alkenyl groups include octyl, dodecyl, hexadecyl, octadecyl, docosanyl, octenyl, dodecenyl, hexadecenyl and octadecenyl.

Phospholipids may be prepared synthetically or derived from natural sources. Synthetic phospholipids may be prepared by methods known to those in the art. Naturally derived phospholipids are often extracted by procedures known to those in the art. Phospholipids may be derived from animal or vegetable sources. A useful phospholipid is derived from sunflower seeds. The phospholipid typically contains 35% to 60% phosphatidylcholine, 20% to 35% phosphatidylinositol, 1% to 25% phosphatidic acid, and 10% to 25% phosphatidylethanolamine, wherein the percentages are by weight based on the total phospholipids. The fatty acid content may be 20% by weight to 30% by weight palmitic acid, 2% by weight to 10% by weight stearic acid, 15% by weight to 25% by weight oleic acid, and 40% by weight to 55% by weight linoleic acid.

Friction modifiers may include fatty amines, esters such as borated glycerol esters, fatty phosphites, fatty acid amides, fatty epoxides, borated fatty epoxides, alkoxylated fatty amines, borated alkoxylated fatty amines, metal salts of fatty acids, or fatty imidazolines, condensation products of carboxylic acids and polyalkylene-polyamines.

In one embodiment the lubricating composition may contain phosphorus- or sulphur-containing antiwear agents other than compounds described as an extreme pressure agent of the amine salt of a phosphoric acid ester described above. Examples of the antiwear agent may include a non-ionic phosphorus compound (typically compounds having phosphorus atoms with an oxidation state of +3 or +5), a metal dialkyldithiophosphate (typically zinc dialkyldithiophosphates), a metal mono- or di-alkylphosphate (typically zinc phosphates), or mixtures thereof.

The non-ionic phosphorus compound includes a phosphite ester, a phosphate ester, or mixtures thereof.

In one embodiment the lubricating composition of the invention further includes a dispersant. The dispersant may be a succinimide dispersant (for example N-substituted long chain alkenyl succinimides), a Mannich dispersant, an ester-containing dispersant, a condensation product of a fatty hydrocarbyl monocarboxylic acylating agent with an amine or ammonia, an alkyl amino phenol dispersant, a hydrocarbyl-amine dispersant, a polyether dispersant or a polyetheramine dispersant.

In one embodiment the succinimide dispersant includes a polyisobutylene-substituted succinimide, wherein the polyisobutylene from which the dispersant is derived may have a number average molecular weight of 400 to 5000, or 950 to 1600.

Succinimide dispersants and their methods of preparation are more fully described in U.S. Pat. Nos. 4,234,435 and 3,172,892.

Suitable ester-containing dispersants are typically high molecular weight esters. These materials are described in more detail in U.S. Pat. No. 3,381,022.

In one embodiment the dispersant includes a borated dispersant. Typically the borated dispersant includes a succinimide dispersant including a polyisobutylene succinimide, wherein the polyisobutylene from which the dispersant is derived may have a number average molecular weight of 400 to 5000. Borated dispersants are described in more detail above within the extreme pressure agent description.

Dispersant viscosity modifiers (often referred to as DVMs) include functionalised polyolefins, for example, ethylene-propylene copolymers that have been functionalized with the reaction product of maleic anhydride and an amine, a polymethacrylate functionalised with an amine, or esterified styrene-maleic anhydride copolymers reacted with an amine may also be used in the composition of the invention.

Corrosion inhibitors include 1-amino-2-propanol, octylamine octanoate, condensation products of dodecenyl succinic acid or anhydride and/or a fatty acid such as oleic acid with a polyamine.

Metal deactivators include derivatives of benzotriazoles (typically tolyltriazole), 1,2,4-triazoles, benzimidazoles, 2-alkyldithiobenzimidazoles or 2-alkyldithiobenzothiazoles. The metal deactivators may also be described as corrosion inhibitors.

Foam inhibitors include copolymers of ethyl acrylate and 2-ethylhexyl acrylate and optionally vinyl acetate.

Demulsifiers include trialkyl phosphates, and various polymers and copolymers of ethylene glycol, ethylene oxide, propylene oxide, or mixtures thereof.

Pour point depressants including esters of maleic anhydride-styrene, polymethacrylates, polyacrylates or polyacrylamides.

Seal swell agents including Exxon Necton-37™ (FN 1380) and Exxon Mineral Seal Oil™ (FN 3200).

Preferably the lubricating oil composition contains co-solvents selected from the group consisting of di-isodecyl adipate, di-propyladipate, di-isotridecyl adipate, trimethylpropyl tricaprylate, diisooctyl adipate, di-ethylhexyl adipate and d-inonyl adipate. Preferably the lubricating oil composition contains co-solvents in an amount of ≧0.5% to ≦35% by weight, more preferably ≧1% to ≦30% by weight, related to the overall weight of the lubricating oil composition.

In another embodiment, the presently claimed invention is directed to a method for reducing the friction coefficient of a lubricating oil composition in the lubrication of a mechanical device comprising formulating said lubricating oil composition with at least one polyalkylene glycol esters of general formula (I) as defined above or a mixture of polyalkylene glycol esters of general formula (I) as defined above.

The friction-modification properties are determined by measuring the friction coefficient at 0-25% slide roll ratio (SRR), mean speed of 4 m/s, using mini-traction machine (MTM) measurements at 70° C. and 1 GPa loading. Reducing the friction coefficient means in the sense of the presently claimed invention that the friction coefficient of a lubricating oil composition comprising a carboxylic acid ester as defined above is lower than the friction coefficient of a lubricating oil composition that does not contain said carboxylic acid ester.

In another embodiment, the presently claimed invention is directed to a method of enhancing the friction modification properties of a lubricating oil composition in the lubrication of a mechanical device comprising formulating said lubricating oil composition with at least one polyalkylene glycol esters of general formula (I) as defined above.

Enhancing the friction-modification properties means in the sense of the present invention that the friction coefficient of a lubricating oil composition comprising a carboxylic acid ester as defined above is lower that the friction coefficient of a lubricating oil composition that does not contain said carboxylic acid ester. The friction-modification properties are determined by measuring the friction coefficient at 25% slide roll ratio (SRR) using mini-traction machine (MTM) measurements at 70° C. and 1 GPa.

A mechanical device in the sense of the presently claimed invention is a mechanism consisting of a device that works on mechanical principles.

The mechanical device is preferably selected from the group consisting of bearings, gears, joints and guidances. Preferably the mechanical device is operated at temperatures in the range of ≧−80° C. to ≦500° C., most preferably in the range of ≧−80° C. to ≦250° C.

EXAMPLES Methods

OHZ=hydroxyl number, determined according to DIN 53240

Mn=number average molecular weight, determined according to DIN 55672-1 and referred to Polystyrene calibration standard.

Mw=weight average molecular weight, determined according to DIN 55672-1 and referred to Polystyrene calibration standard.

PD=polydispersity, determined according to DIN 55672-1

SZ=acid number, determined according to ASTM D664.

Measuring Physical Properties

The kinematic viscosity was measured according to the standard international method ASTM D 445.

The viscosity index was measured according to the ASTM D 2270.

The pour point according was measured to DIN ISO 3016.

Friction Coefficient Evaluation

The fluids were tested in the MTM (Mini-Traction Machine) instrument using the so-called traction test mode. In this mode, the friction coefficient is measured at a constant mean speed over a range of slide roll ratios (SRR) to give the traction curve. SRR=sliding speed/mean entrainment speed=2 (U1−U2)/(U1+U2) in which U1 and U2 are the ball and disc speeds respectively

The disc and ball used for the experiments were made of steel (AISI 52100), with a hardness of 750 HV and Ra<0.02 μm. The diameter was 45.0 mm and 19.0 mm for the disc and the ball respectively. The tractions curves were run with 1.00 GPa contact pressure, 4 m/s mean speed and 70° C. temperature. The slide-roll ratio (SRR) was varied from 0 to 25% and the friction coefficient measured.

Oil Compatibility Evaluation

A method was developed in-house to determine oil compatibility. The oil and test material were mixed in 10/90, 50/50 and 90/10% w/w ratios respectively. The mixtures were mixed at room temperature by rolling for 12 hours. The mixtures' appearance was observed after homogenization and again after 24 hours. The test material is deemed compatible with the oil when no phase separation is observed after 24 hours for at least two of the ratios investigated.

Synthesis of the Polyalkylene Glycols Example 1

Polytetrahydrofurane (pTHF 250; MW 250.0 g/mol, 187 g, 0.75 mol) was mixed with isostearic acid (302.7 g/mol; 454.1 g, 1.5 mol) and heated to 180° C. under nitrogen flow for 15 h until an acid number of 5.1 mg KOH/g is reached. Thereby 25 ml condensate water is obtained. Then tin (118 g/mol; 3.13 g, granules) is added. The reaction mixture is heated again to 180° C. and held at this temperature for 48 h. The acid number of the crude product had an acid number of 0.7 mg KOH/g. The reaction mixture was allowed to cool to 25° C. and 2.0 ml water and powdered sodium hydrogen carbonate (50 g) was added. The mixture was stirred for 12 h and press filtered (Filter Type Seitz K900). The final product was characterized:

Yield: 0.523 kg (99% of Theory)

Acid number SZ: 0.7 mg KOH/g=0.5% isostearic acid; 99.5% turnover;

GPC: Mn: 1121; Mw: 1168; D: 1.04

Example 2

Polytetrahydrofurane (pTHF 650; MW 650.0 g/mol, 905 g, 1.39 mol) was mixed with isostearic acid (302.7 g/mol; 843.2 g, 2.78 mol) and heated to 220° C. under nitrogen flow for 15 h until an acid number of 8.9 mg KOH/g. Thereby 50 ml condensate is obtained. Then tin octanoate (405 g/mol; 4.03 g, 10 mmol) is added. The reaction mixture is heated again to 220° C. and held at this temperature for 48 h. The acid number of the crude product had an acid number of 3.9 mg KOH/g. The reaction mixture was allowed to cool to 25° C. and 2.5 ml water and powdered sodium hydrogen carbonate (260 g) was added. The mixture was stirred for 12 h and press filtered (Filter Type Seitz K150). The final product was characterized:

Yield: 1.5 kg (99% of Theory)

Acid number SZ: 2.0 mg KOH/g=2.2% isostearic acid; 97.8% turnover;

GPC: Mn: 1578; Mw: 2104; D: 1.33

Example 3

Polytetrahydrofurane (pTHF 1000; MW 1000 g/mol, 500 g, 1.00 mol) was mixed with isostearic acid (302 g/mol; 302.7 g, 2.00 mol) and heated to 180° C. under nitrogen flow for 8 h until an acid number of 10.6 mg KOH/g. Thereby 14 ml condensate obtained. Then 0.5 g para-toluene sulfonic acid and tin powder (405 g/mol; 4.03 g, 5 mmol) is added. The reaction mixture is heated to 180° C. and held at this temperature for 21 h. The acid number of the crude product had an acid number of 0.1 mg KOH/g. The reaction mixture was allowed to cool to 25° C. and powdered sodium hydrogen carbonate (50 g) was added. The mixture was stirred for 12 h and press filtered (Filter Type Seitz 900). The final product was characterized:

Yield: 0.789 kg (99% of Theory)

Acid number SZ: 0.1 mg KOH/g, 99.9% turnover

GPC: Mn: 1940; Mw: 3070; D: 1.58

Example 4 (Comparative)

Polytetrahydrofurane (pTHF 650; MW 650.0 g/mol, 650 g, 1.00 mol) was mixed with 2-ethylhexanoic acid (144.2 g/mol; 288.4 g, 2.00 mol) and heated to 200° C. under nitrogen flow for 18 h until an acid number of 18.7 mg KOH/g. Thereby 17 ml condensate obtained. Then tin octanoate (405 g/mol; 4.03 g, 5 mmol) is added. The reaction mixture is heated to 240° C. and held at this temperature for 6 h. The acid number of the crude product had an acid number of 1.1 mg KOH/g. The reaction mixture was allowed to cool to 25° C. and powdered sodium hydrogen carbonate (150 g) was added. The mixture was stirred for 12 h and press filtered (Filter Type Seitz K150). The final product was characterized:

Yield: 0.9 kg (98% of Theory)

Acid number SZ: 0.6 mg KOH/g=0.5% 2-ethylhexanoic acid; 99.5% turnover;

GPC: Mn: 1380; Mw: 1950; D: 1.41

Example 5 (Comparative)

Polytetrahydrofurane (pTHF 650; MW 650.0 g/mol, 650 g, 0.50 mol) was mixed with stearic acid (284.5 g/mol; 284.5 g, 1.0 mol) and heated to 220° C. under nitrogen flow for 92 h until an acid number of 4.08 mg KOH/g. Thereby 17 ml condensate obtained. Then tin octanoate (405 g/mol; 1.82 g, 5 mmol) is added. The reaction mixture is heated to 220° C. and held at this temperature for 48 h. The acid number of the crude product had an acid number of 0.97 mg KOH/g. The reaction mixture was allowed to cool to 90° C. and 200 ml toluene was added to dissolve the product. Powdered sodium hydrogen carbonate (230 g) was added. The mixture was stirred for 12 h at 25° C. and press filtered (Filter Type Seitz K900). Toluene was removed from the filtrate at 60° C./10 mbar. The final product was characterized:

Yield: 0.553 kg (98% of Theory)

Acid number SZ: 0.9 mg KOH/g=1.05% isostearic acid; 99% turnover;

Example 6 (Comparative)

Polybutylene oxide (MW 1242.0 g/mol, 621 g, 0.50 mol) was mixed with isostearic acid (284.4 g/mol; 285.7 g, 1.0 mol) and heated to 220° C. under nitrogen flow for 20 h until an acid number of 7.2 mg KOH/g was reached. Thereby 30.2 ml condensate is obtained. Then tin octanoate (405 g/mol; 1.2 g, 3 mmol) is added. The reaction mixture is heated again to 220° C. and held at this temperature for 8 h. The acid number of the crude product had a value of 0.1 mg KOH/g. The final product was characterized:

Yield: 0.85 kg (99% of Theory)

Acid number SZ: 0.1 mg KOH/g=0% isostearic acid; 99.9% turnover;

Results

The oil compatibility and friction data are summarized in Table 1. The data demonstrate that the molecules derived from the present invention (Example 1, 2 and 3), namely polyalkylene glycol esters produced by reacting polytetrahydrofuran (p-THF) with isostearic acid (C₁₈) show compatibility with mineral oils and low viscosity polyalphaolefins whilst providing low friction coefficients (≦0.015 at 25% SRR in MTM experiments).

TABLE 1 MTM Mineral oil Group III Low viscosity PAO Kinematic friction compatibility at compatibility at viscosity Pour coefficient room temperature room temperature (mm²/s) Viscosity point at (oil/test material) (oil/test material) 40° C. 100° C. Index (° C.) 25% SRR 10/90 50/50 90/10 10/90 50/50 90/10 Example 1 58 11 180 −12 −12 0.013 Yes Yes Yes Yes Yes Yes Example 2 138 23 198 −3 −3 0.009 Yes Yes Yes Yes Yes Yes Example 3 246 39 210 −3 −3 0.008 No Yes Yes No Yes Yes Example 4 85 16 196 −33 −33 0.010 No No Yes No No Yes (comparative) Example 5 [*] n/a [*] [*] 0.010 Yes [*] [*] No [*] [*] (comparative) Example 6 132 18 156 −48 −48 0.030 Yes No No Yes No No (comparative) [*] material solid at room temperature and 40° C. 

1.-15. (canceled)
 16. A lubricant which comprises a polyalkylene glycol ester of the formula (I)

wherein k is an integer in the range of ≧2 to ≦30, and R₁ and R₂, identical or different, each denote an unsubstituted, branched alkyl radical having 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or 26 carbon atoms.
 17. The lubricant according to claim 16, wherein k is an integer in the range of ≧3 to ≦25.
 18. The lubricant according to claim 16, wherein the polyalkylene glycol ester of the formula (I) has a weight average molecular weight Mw in the range of ≧500 to ≦4500 g/mol, determined according to DIN 55672-1 (polystyrene calibration standard).
 19. The lubricant according to claim 16, wherein the polyalkylene glycol ester of the formula (I) has a number average molecular weight Mn in the range of ≧400 to ≦4000 g/mol, determined according to DIN 55672-1 (polystyrene calibration standard).
 20. The lubricant according to claim 16, wherein the polyalkylene glycol ester of the formula (I) has a polydispersity in the range of ≧1.1 to ≦2.0, determined according to DIN 55672-1 (polystyrene calibration standard).
 21. The lubricant according to claim 16, wherein R₁ and R₂, identical or different, each denote a moiety selected from the group consisting of

whereby n is an integer in the range of ≧8 to ≦23,

whereby m is an integer in the range of ≧7 to ≦22,

whereby o is an integer in the range of ≧1 to ≦6, p is an integer in the range of ≧2 to ≦19, and (o+p) is an integer in the range of ≧8 to ≦21, and

whereby Q is an integer in the range of ≧1 to ≦6, R is an integer in the range of ≧2 to ≦19, and (Q+R) is an integer in the range of ≧8 to ≦20.
 22. The lubricant according to claim 16, wherein R₁ and R₂ are identical.
 23. The lubricant according to claim 16, wherein the polyalkylene glycol ester of the formula (I) has a total acid number in the range of ≧0.01 to ≦2.5 mg KOH/g, determined according to ASTM D664.
 24. The lubricant according to claim 16, wherein the polyalkylene glycol ester of the formula (I) has a hydroxyl number in the range of ≧0.1 to ≦20 mg KOH/g, determined according to DIN
 53240. 25. The lubricant according to claim 16, wherein k is an integer in the range of ≧3 to ≦25, and R¹ and R², identical, each denote a moiety selected from the group consisting of

whereby n is 10, 11, 12, 13, 14, 15, 16, 17 or
 18. 26. A lubricating oil composition comprising at least one polyalkylene glycol ester of the the formula (I) as defined in claim
 16. 27. A method for reducing the friction coefficient of a lubricating oil composition in the lubrication of a mechanical device comprising formulating said lubricating oil composition with at least one polyalkylene glycol ester of the formula (I)

wherein k is an integer in the range of ≧2 to ≦30, and R₁ and R₂, identical or different, each denote an unsubstituted, branched alkyl radical having 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or 26 carbon atoms.
 28. The method according to claim 27, wherein the friction coefficient is measured at 0-25% slide roll ratio (SRR), mean speed 4 m m/s, using mini-traction machine (MTM) measurements at 70° C. and 1 GPa loading.
 29. A method of enhancing the friction modification properties of a lubricating oil composition in the lubrication of a mechanical device comprising formulating said lubricating oil composition with at least one polyalkylene glycol ester of formula (I)

wherein k is an integer in the range of ≧2 to ≦30, and R₁ and R₂, identical or different, each denote an unsubstituted, branched alkyl radical having 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or 26 carbon atoms. 